Method and apparatus for liquefied gas storage



Dec. 31, 1968 G. E. ENGDAHL 3,419,174

METHOD AND APPARATUS FOR LIQUEFIED GAS STORAGE Filed July 18, 1963 Sheet of 2 heya Dec. 31, 1968 G. E. ENGDAHI. 3,419,174

METHOD AND APPARATUS FOR LIQUEFIED GAS STORAGE Filed July 18, 1965 Sheet 2 of 2 m LLI/?" Wai Hmm Q5 g jl; l @5 zfy United States Patent() 3,419,174 METHOD AND APPARATUS FOR LIQUEFIED GAS STORAGE Gerald E. Eugdahl, Glen Ellyn, Ill., assignor to Chicago Bridge & Iron Company, Oak Brook, Ill., a corporation of Illinois Filed July 18, 1963, Ser. No. 295,975 12 Claims. (Cl. 220-9) This invention relates to insulated cryogenic storage tanks. More particularly, this invention is concerned with improved cryogenic tanks which have low heat-leak characteristics and which are more economical to construct than other cryogenic tanks of comparable size.

A number of gases are stored in liquefied condition. Among these gases are oxygen, hydrogen, natural gas including methane, ethane, ethylene, carbon dioxide and nitrogen.

One type of cryogenic tank for storing such gases in liquefied form is comprised of an inner shell of metal which remains ductile at the extremely low temperatures of the liquefied gas. The inner shell quite often has a substantially flat metal bottom, perpendicular wall and a domed roof. The bottom is usually circular and the wall usually cylindrical. Surrounding the inner shell is an outer shell having a bottom, side wall and domed roof. Low temperature ductility of the metal is not required for the outer shell. Between the inner and outer shell bottoms is insulation such as perlite, plastics or light-weight concrete. The space between the inner and outer shell walls, as well as between the roof shells, is filled with a suitable insulating material such as perlite.

Vessels having such double-shelled roofs separated by insulation are expensive to build and have other disadvantages. One of these is that considerable heat-leak takes place from the internal shell portion above the level of the liquid cryogen in the tank. The unwetted portion of the inner shell wall constitutes a serious heat-leak source from the external atmosphere through the insulation to the inner shell and down to the liquid and greatly affects the pressure conditions in the vapor space in the tank above the liquid level of the cryogen. The Vapor space of the tank is designed to be at a low temperature. The large heat leak through the tank raises the temperature of the vapor and increases the pressure. To maintain a safe pressure in the tank it is necessary to continuously vent vapor from the tank.

In large tanks, the mechanism of heat transfer has been observed to be such that the vapor space reacts primarily to heat leak and pressure change as if it were not in the presence of the liquid cryogen. Pressure in a conventional insulated tank primarily is determined, not by the temperature of the liquid cryogen whether subcooled or not, but by the heat leak through the tank which results in the production of a vapor pressure considerably higher than would be otherwise expected from the temperature of the liquid itself. In other Words, the heat exchange from outside of the tank to the vapor space is greater than from the vapor space to the liquid cryogen so that it is the heat exchange through the tank from outside thereof to the vapor space which substantially determines the pressure in the tank. As a result, pressure changes in such tanks may be predicted by merely applying the gas laws to the vapor space.

Because of the large differential temperature between the atmosphere and the low temperature in the vapor space of conventional tanks heat leak is pronounced. This heat leak can produce a rather rapid rate of pressure rise in the tank. This necessitates considerable venting in tanks designed for only about 1 p.s.i. vapor pressure.

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The use of such conventional tanks for storing liquefied gases in a subcooled condition also presents the potential hazard of collapse of the tank because of a reduction in the vapor pressure due to condensation of vapor by the subcooled liquid. Thus, in a conventional tank as described methane can be stored at 260 F. at a vapor pressure of l5 p.s.i.a. At this temperature the vapor pressure is balanced against the external air pressure. However, if the methane is cooled to 280 F., the vapor commences to condense and such condensation reduces the vapor pressure to approximately l0 p.s.i.a. at which the roof would collapse unless it was specially reinforced or breathing were permitted.

It has been found, according to the subject invention, that the storage in a tank of a cryogenic liquid at about ambient atmospheric pressure and at a temperature of about equilibrium temperature or a subcooled temperature can be effected by placing a floating thermal barrier or cover upon the surface of the liquid within the tank so as to greatly reduce the exchange of heat between the vapors in the tank and the cryogenic liquid. The floating thermal barrier permits the storage of a cryogenic liquid in a subcooled condition without reducing the vapor pressure in the vapor space above the liquid to a dangerous lev'el that might lead to collapse of the roof or upper shell of the tank. In the case of methane, by greatly reducing the surface of liquid that is exposed to the vapor, it can be stored subcooled, such as at *280 F., without significantly reducing the vapor pressure in the vapor space.

It has been found further that by placing a thermal barrier, such as an insulated floating cover, inside the tank on the surface of the cryogenic liquid, heat leak through the unwetted portion of the inner shell is greatly reduced because the vapor space can be kept at or near ambient temperature. As a result it is possible to employ a noninsulated tank roof, such as a single metal shell, projected to the tank wall, in place of the expensive insulated, double-shelled roof structure used on conventional cryogenic storage tanks although the latter can be used, if desired, in combination, with the floating thermal barrier. Also, since the vapor space contains a warm light Vapor, whose temperature is about that of the ambient air, and because heat leak is minimal the amount of venting is much less than in the conventional tanks.

A representative cyrogenic storage tank according to the invention comprises an enclosed tank having an insulated botto-m, a substantially vertical insulated wall projecting upwardly therefrom with an essentially noninsulated roof joined to the wall and completely covering the space surrounded `by the wall and an insulated floating cover inside the tank, said cover having a surface shape complementary to a horizontal section of the interior of the tank with the peripheral edge of the cover being adjacent the wall surface inside the tank to permit free vertical movement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid in the tank.

The invention will now be described in conjunction with the attached drawings in which:

FIGURE l is a cross-sectional elevation of a cryogenic tank according to the subject invention;

FIGURE 2 is a cross-sectional elevation of a cyrogenic tank showing a floating cover insulated differently than in FIGURE l;

FIGURE 3 is a vertical cross section through the edge of a floating cover in a cyrogenic tank illustrating an a1- ternative construction of the floating cover;

FIGURE 4 is a cross-sectional elevation of a cryogenic tank showing a composite insulated tank containing a floating cover;

FIGURE 5 is a cross-sectional elevation of a cryogenic tank showing foam insulation inside the tank shell; and

FIGURE 6 is a cross-sectional elevation of a cryogenic tank showing still another way to insulate the tank interior containing the fioating cover.

Identical elements which appear in more than one gure of the drawings will be numbered identically unless otherwise indicated.

The cryogenic storage tank of FIG. 1 comprises an inner metal shell 10 which is made up of a bottom plate 11 and side shell 12. The side shell 12 is joined at the bottom 11 by suitable means, such as welding, so as to provide an inner shell capable of holding a liquid cryogen without leakage.

Surrounding the inner shell 10 is outer shell 13 which has a bottom plate 14 and a side shell 15 which is joined to the bottom 14 by suitable means such as weldin g. The outer shell 13 has a roof shell 16 which is shaped like a dome or an inverted dish. The roof shell 16 is joined to the side in such a manner as to provide a completely closed tank.

In the space between the outer bottom shell 14 and the inner bottom shell 11 is placed a suitable insulation 17 which can withstand the Weight of the inner tank and of the liquid cryogen to be stored in the tank. Insulation 18 is placed between the inner shell 12 and the outer shell 15. The insulation 18 need not be a load-bearing insulation since it is not subjected to the high pressures applied against insulation 17. Conduit 19 is provided to fill the tank with a liquid cryogen or to remove it therefrom.

The inner side shell 12 terminates at a distance slightly below the roof shell 16. The space `between the inner side shell 12 and the outer side shell 15 at the upper edge 21 of the inner shell 12 can be kept open to the inside of the tank. Pads of insulation 20 are provided at such area to retain granular insulation 18 in place and to prohibit vapor currents from Ablowing the same about the interior of the tank.

Although it would be possible to join the top edge 21 of the inner shell 12 to the outer shell 15, or to the roof shell 16, by a plate in the form of a ring extending around the inside of the tank, it is not desirable to do this because such a plate would provide a heat-leak source and would also make it necessary to provide ports therein or in the inner side shell 12 so that the vapor pressure between shells 12 and 15 can uctuate with barometric pressure and temperature. Unless the pressure between shells 12 and 15 can uctuate, the tank shells could buckle or break through creation of a large differential fbetween the vapor pressure in the tank and the pressure between the shells.

The roof shell 16 is self-supporting under the conditions of use and is made of a material that can withstand the Vapor pressure inside of the tank. For the purpose of this invention, the domed roof shell 16 can `be made of a noninsulated mild steel plate.

Inside of the tank there is provided a floating cover 23. It has a 'bottom shell 24 which has a vertical upstanding side 25 at the peripheral edge thereof. The floating cover is complementary in area and shape to the inside horizontal cross-sectional area of the inner shell 10. The side 25 of the floating cover is substantially adjacent to the inside surface of shell 10. The clearance between the side 2S and the inside of `shell 10 is sufficient to provide clearance and free movement of the floating cover as the level of the liquid cryogen in the tank goes up or down. The bottom 24 and side 25 can be metal.

The floating cover 23 has an upper horizontal shell 26, made of metal or plastic, which extends over the full area of the cover and is approximately identical in size to the cover bottom shell 24. An opening 27 is provided in the iioating cover so that vapor from the liquid cryogen can be vented into the vapor space 22. Insulating material 28 is placed in the floating cover 23 between the bottOm Shell 24 and the top shell 26. Any suitable insulating material, such as perlite, can be used.

A conduit 29 is provided -which communicates with the vapor space 22. By means of conduit 29' vapor can be removed from the vapor space 22 when the pressure becomes higher than desired. The vapor removed can be compressed, liquefied and pumped back into the tank or, if the cryogen is stored at a subcooled temperature, the vapor can be compressed and circulated "directly into the liquid cryogen where the vapor will condense and be liquefied. The vapor can also be compressed and sent to a distribution system.

Although a tank such as shown in FIGURE 1 can be of various cross-sectional shapes, it is usually circular and therefore the floating roof -will also be of circular shape.

The insulating effect provided by floating cover 23 against heat transfer between the liquid cryogen and the outside area of the tank, as well as to the vapor space 22, makes it unnecessary to insulate the roof shell 16. The roof shell 16 can be maintained at ambient temperature and thus it need not be constructed of materials which can withstand the low temperatures of liquid cryogens. Mild steel plates can be used for the roof shell 16. There are also savings in material by elimination of the insulation otherwise required in such a roof, as well as the cost in fabricating and constructing a double-shelled insulated roof structure.

Referring now to FIGURE 2, in which the parts are the same as in FIGURE l unless otherwise stated, there is shown a cryogenic tank according to the subject invention having -iioating cover 30 on a liquid cryogen 31. The iioating cover has a metal bottom sheet 32 that has an upright or vertical side 33 at its peripheral edge. The iioating cover 30 is more or less in the form of a shallow pan which holds insulating material 34, particularly of a granular composition uniformly distributed on the upper side of the metal bottom between the vertical sides. To avoid disturbing the granular insulation, such as by vapor currents, a layer of insulation 35 which is not susceptible to ready movement by vapor current or other conditions to which it will be subjected in the tank is placed at the top. Slabs of glass wool or foamed plastic can be used for this purpose.

In FIGURE 3 there is shown in section the peripheral edge portion of a floating cover 40. It is composed of a bottom shell 41 and peripheral vertical side shell 42 joined to the bottom or made as an integral part thereof. The shell portions 41 and 42 can be made of a suitable plastic material which can withstand the temperatures to which it is subjected by contacting the liquid cryogen in the tank. It can be made of an epoxy or polyurethane material having embedded therein a suitable substance, such as fiber glass, for reinforcing and stiifening purposes. A suitable insulating material 43 is placed in the cover shell made of bottom 41 and side `42. The insulation 43 can be any of the conventional materials used for insulating purposes in cryogenic tanks. In particular, it can be a foamed plastic such as polyurethane foam. The plastic foam can be foamed right in the shell or it can be prefoamed, cut into slabs and the slabs used in fabricating the cover.

The tank of FIGURE 5 has a metal bottom 14, side shell 15 and roof 16. The inside of the bottom and sides are insulated by suitable load bearing plastic foam 45, such as polyurethane foam, which can be foamed in place with the necessary thickness. The iioating cover 46 is also plastic foam and as shown in the drawing is one piece; however, it can be fabricated of a plurality of pieces of foam.

The tanfk in FIGURE 4 is like that in FIGURE 5 except that the plastic foam insulation 45 is covered with a suitable barrier of plastic material 47 such as fiber glass embedded in a plastic such as an epoxy. In FIGURE 4 a barrier layer such as fiber glass embedded in plastic serves to retard penetration of the vapors and liquid cryogen into the plastic foam which may cause it to expand upon pressure drop. The barrier material 47 extends over the top edge 48 of the plastic foam and into contact with the tank shell wall 15. The barrier 47 can be bonded to insulation 45. Floating cover 46 of foamed plastic is the same in FIGURES 4 and 5.

The tank of FIGURE 6 has self-supporting load bearing insulation S0 at the inside of the bottom and sides of the metal shell. A manhole 51 is provided for access into the tank. An annular angle element 52 is attached to the inside of the roof 16. A ange 53, which is part of element 52, projects downwardly and extends around the roof as a ring located about above the inner vertical wall of the insulation 50. Suspending from flange S3 is a fiexible membrane 54 having side walls and a bottom. The top peripheral edge of the membrane 54 is removably secured to ange 53 by retaining rim 55. The membrane is of such size as to be in essentially continuous but removable contact with the bottom and walls of insulation 50. The fiange arrangement 56 serves to tie the membrane to pipe 19 to prevent liquid cryogen from leaking past the membrane into the insulation 50. The membrane serves as a barrier to keep the liquid cryogen from penetrating insulation 50. The floating cover in the tank of FIGURE 6 has a plastic foam core 57 covered with a suitable barrier material 58 such as fiber glass embedded in plastic or a rubber film.

Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.`

What is claimed is:

1. An enclosed cryogenic storage tank comprising an insulated bottom with a substantially vertical insulated wall projecting upwardly therefrom around the periphery of the Ibottom, a noninsulated metal roof projecting to the wall and completely covering the space surrounded by the wall, said roof having a vapor opening an insulated floating cover inside the tank, said cover having a surface shape complementary to the interior of the tank in horizontal section with the peripheral edge of the cover being adjacent the wall surface inside the tank to permit free lvertical movement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid lwhen placed in the tank, and an opening in the tank for filling the tank with a cryogenic liquid.

2. The method of storing a cryogenic liquid which comprises placing a cryogenic liquid in an enclosed storage tank that is insulated at least to the le-vel of the liquid stored therein and placing a floating thermal barrier inside the tank upon the 'surface of said liquid to reduce the surface area of said liquid in contact with vapor in the tank and thereby reduce heat exchange between the liquid and vapor and permit the vapor space to be at about ambient temperature.

3. The method of storing a cryogenic liquid which comprises placing a cryogenic liquid in an enclosed storage tank at ambient atmospheric pressure and at a temperature no higher than the equilibrium temperature for such liquid at ambient pressure, said tank being insulated to the level of the liquid stored therein but having a noninsulated metal roof, placing a floating thermal barrier inside the tank upon the surface of said liquid to reduce the surface area of said liquid in contact with vapor in the tank and thereby reduce the heat exchange between the liquid and vapor.

4. An enclosed cryogenic storage tank comprising an insulated bottom having an outer metal layer and an inner barrier layer with insulation therebetween, a substantially vertical insulated wall projecting upwardly from and around the periphery of the bottom, said wall having an inner barrier layer and an outer metal shell with insulation therebetween, a noninsulated metal roof shell projecting to the outer metal shell of the wall and completely covering the space surrounded by the Wall, said roof having a vapor opening, an insulated floating cover inside the tank, said cover having a surface sha-pe complementary to the interior of the tank in horizontal section with the peripheral edge of the cover beinig adjacent the wall surface inside the tank to permit free vertical movement of the cover while fioating on, and covering substantially the total surface of, a cryogenic liquid when placed in the tank, and an opening in the tank for filling the tank with a cryogenic liquid.

5. A storage tank according to claim 4 in which the space between the inner barrier layer and outer metal shell of the wall is open to the vapor space of the tank at a height above the maximum level to which the tank is filled with a cryogenic liquid.

6. An enclosed cryogenic storage tank comprising an insulated bottom with a substantially vertical insulated wall projecting upwardly therefrom and around the periphery of the bottom, said wall having an inner and outer metal shell with insulation therebetween, an essentially noninsulated metal roof shell projecting to the outer shell of the wall and completely covering the space surrounded by the wall, said roof having a vapor opening, an insulated floating cover inside the tank, said cover having a surface shape complementary to the interior of the tank in horizontal section with the peripheral edge of the cover being adjacent the wall surface inside the tank to permit free vertical movement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid when placed in the tank, and an opening in the tank for filling the tank with a cryogenic liquid.

7. An enclosed cryogenic storage tank comprising a metal bottom having insulation on the upper side with a substantially vertical metal wall having insulation on the inner side projecting upwardly therefrom around the periphery of the bottom, an essentially noninsulated metal roof projecting to the `wall and completely covering the space surrounded by the wall, said roof having a vapor opening, a removable barrier layer placed over and against the surface of the insulation on the bottom and vertical wall, an insulated floating cover inside the tank, said cover having a surface shape complementary to the interior of the tank in horizontal section with the peripheral ed-ge of the cover being adjacent the wall surface inside the tank to permit free vertical movement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid when placed in the tank, and an opening in the tank for rfilling the tank with a cryogenic liquid.

8. An enclosed cryogenic storage tank comprising a rnetal bottom having insulation on the upper side with a substantially vertical metal wall having insulation on the inner side projecting upwardly therefrom around the periphery of the bottom, an essentially noninsulated metal roof projecting to the vertical wall and completely covering the space surrounded by the wall, said roof having a vapor opening, a fiexible removable barrier layer suspended from the lower side face of the tank roof in spaced relation to said vertical wall and placed over and against the surface of the insulation on the bottom and vertical wall, and an insulated oating cover inside the tank, said cover having a surface shape complementary to the interior of the tank in horizontal section with the peripheral edge of the cover being adjacent the `wall surface inside the tank to permit free vertical movement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid in the tank, and an opening in the tank for filling the tank with a cryogenic liquid.

9. A storage tank according to claim 8 in which the floating cover comprises insulating material contained in a barrier layer which surrounds the same and forms the outer cover surface.

10. An enclosed cryogenic storage tank comprising an innsulated bottom having an outer metal layer and an inner barrier layer with insulation therebetween, a substantially vertical insulated wall projecting upwardly from and around the periphery of the bottom, said wall having an inner barrier layer and an outer metal shell with insulation therebetween, a metal roof shell projecting to the outer metal shell of the wall and completely covering the space surrounded by the Wall, said roof having a vapor opening, an insulated floating cover inside the tank, said cover having a surface shape complementary to the interior of the tank in horizontal section with the peripheral edge of the cover being adjacent the wall surface inside the tank t0 permit free vertical movement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid when placed in the tank, and an opening in the tank for lling the tank with a cryogenic liquid.

11. An enclosed cryogenic storage tank comprising an insulated bottom with a substantially vertical insulated wall projecting upwardly therefrom and around the periphery of the bottom, said wall having an inner and outer metal shell with insulation therebetween, an essentially noninsulated metal roof shell and projecting to and joining the outer shell of the 'wall and completely covering the space surrounded by the Wall, the top edge of the inner shell being spaced -below and out of Contact with the underside of the roof, the space between the inner and outer metal shells of the wall being open to the vapor space of the tank at a height above the maximum level to which the tank is filled with a cryogenic liquid, said roof having a vapor opening, an insulated oating cover inside the tank, said cover having a surface shape complementary to the interior of the tank in horizontal section with the peripheral edge of the cover being adjacent the wall surface inside the tank to permit free vertical lmovement of the cover while floating on, and covering substantially the total surface of, a cryogenic liquid when placed in the tank, and an opening in the tank for filling the tank with a cryogenic liquid.

12. A storage tank according to claim 11 in which the insulation between the inner and outer metal shells is granular and vapor permeable means is provided at the top of the granular insulation to restrain its dislodgement by turbulent ow of vapors in the tank.

References Cited UNITED STATES PATENTS 2,307,508 1/1943 Jayne 220-265 3,049,261 8/ 1962 Wade et al. 220--26 3,163,434 12/1964 Krueger 220-9 280,820 7/1883 Hickson 220-26 1,088,194 2/1914 Whitten 220-9 1,846,554 2/ 1932 Holbrook 220-9 2,460,35 5 2/1949 Kornemann 220-9 2,927,712 3/ 1960 Armato et al. 220-63 2,994,452 8/ 1961 Morrison 220-19 3,085,708 4/1963 Dosker 220-9 3,090,517 5/1963 Morrison 220-9 3,144,167 8/ 1964 Schultz 220-63 3,147,878 9/ 1964 Wissmiller 220-9 3,158,667 11/1964 Michaels 220-26 FOREIGN PATENTS 213,782 2/1961 Austria.

281,681 12/ 1914 Germany. 1,076,342 2/1960 Germany.

THERON E. CONDON, Primary Examiner.

JAMES R. GARRETT, Assistant Examiner.

U.S. Cl. X.R. 2120-26 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,419,174 December 3l,

Gerald E. Engdahl It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 35, "opening" should read opening, Column 6, li 7l, "innsulated" should read insulated Column 8, line 26, 281,681"

Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. AN ENCLOSED ORYOGENIC STORAGE TANK COMPRISING AN INSULATED BOTTOM WITH A SUBSTANTIALLY VERTICAL INSULATED WALL PROJECTING UPWARDLY THEREFROM AROUND THE PERIPHERY OF THE BOTTOM, A NONINSULATED METAL ROOF PROJECTING TO THE WALL AND COMPLETELY COVERING THE SPACE SURROUNDED BY THE WALL, SAID ROOF HAVING A VAPOR OPENING AN INSULATED FLOATING COVER INSIDE THE TANK, SAID COVER HAVING A SURFACE SHAPE COMPLEMENTARY TO THE INTERIOR OF THE TANK IN HORIZONTAL SECTION WITH THE PERIPHERAL EDGE OF THE COVER BEING ADJACENT THE WALL SURFACE INSIDE THE TANK TO PERMIT FREE VERTICAL MOVEMENT OF THE COVER WHILE FLOATING ON, AND COVERING SUBSTANTIALLY THE TOTAL SURFACE OF, A CRYOGENIC LIQUID WHEN PLACED IN THE TANK, AND AN OPENING IN THE TANK FOR FILLING THE TANK WITH A CRYOGENIC LIQUID. 